When maintenance is performed on any machine or apparatus incorporating pipes, there is frequently the need to plug a pipe while work is being performed upstream or downstream of the plug. The plug would then act to hold back a head of water and would prevent any debris or other foreign objects from passing through the pipe into another section of the apparatus. The large scale pipes used in nuclear power plants are known to require such maintenance.
Typically, a plug will be inserted through a manway somewhat smaller than the pipe in which it will be ultimately installed. In such a case it is desirable that the plug be comparatively small initially and expand later once within the pipe. A standard approach, therefore, is to use an inflatable ball or cylinder plug, which is collapsed initially and inflated once installed. Examples of such inflatable plugs are those disclosed in the Van der Lans U.S. Pat. No. 4,079,755 and the Riegel, et al. U.S. Pat. No. 3,842,864.
Of course, it is necessary for the plug, once installed and inflated, to be stable and effect a firm, immovable seal. A problem with conventional inflatable ball or cylinder plugs is that their axial length must generally be equal to or greater than their diameters in order to effect such a seal. As a consequence, it is frequently impractical to use such plugs when bends in the piping or other interference require a relatively short, maneuverable plug. When several plugs are used in series for added protection, the problem becomes even more acute. Special reinforcing or special installation equipment add to the cost of an inflatable plug and may interfere with its maneuverability.